Industrial Useful Field
This invention relates to an improvement of a drive stage control mechanism for automatic transmission.
Prior Art and Its Problem
An automatic transmission, which is mounted on such a vehicle and composed of a torque converter and a planetary gear transmission having plural drive stages, is generally designed to be changed its drive stage by engaging or disengaging components of the planetary gear transmission to or from an input shaft or a housing.
These engagement and disengagement of the components of planetary gear transmission become evident as a feeling at the time of changing speed. If gear components for first speed are disengaged before gear components for second speed are engaged when shifting the drive stage from first speed to second speed, for example, an engine power will be subjected to no load condition to cause a sudden rise of engine rotation.
On the other hand, if the gear components for first speed which are to be disengaged are kept at engaged condition even after the gear components for second speed have been engaged, for example, input and output shafts will be braked so as to produce a loss in efficiency and give uncomfortable feeling to passengers due to instantaneous deceleration.
Further, in case of sudden engagement or disengagement of gear components, they will become evident as a so-called speed-change shock to give uncomfortable feeling to passengers.
As described above, a timing of engagement or disengagement of the gear components is an important factor for deciding the feeling of transmission. Therefore, in order to improve this feeling, such construction has conventionally been employed that a one-way clutch is interposed between an output shaft of converter and a planetary gear transmission utilizing a reversing of force applied on the gear components before and after changing speed, or that an accumulator or an orifice is installed in a hydraulic control circuit for controlling the engagement and disengagement of gear components and is controlled by a timing valve.
In case of the former construction using the one-way clutch, however, sudden engagement will occur and induce the speed-change shock to cause an increase in cost uselessly at the time of shift-down, especially at the time of so-called kick-down.
Further, in case of the latter construction for hydraulic control wherein a load applied on a friction material is to be adjusted only by a hydraulic pressure actuating on a piston, an accurate control becomes impossible due to an influence of masses of the piston and the friction material and its mechanism becomes complicated to cause an increase in manufacturing cost.
There are prior arts (U.S. Pat. No. 3,126,752 and Japan Pat. No. 39-5562) in which a coned disc spring pressing on the friction material is provided and a two-stage piston is adapted to press the friction material, however, the both prior arts are designed to control only engagement of gear components so that both the engagement and disengagement of components can not be controlled.